Teenage tyrannosaurs, 'megatheropods' limited diversity of smaller dinos

For more than 150 million years, dinosaurs dominated Earth's ecosystems and food chains, but for all their dominations, dinosaurs boasted surprisingly low rates of speciation.

Scientists have struggled to explain why dinosaur diversity remained so low. Now, new research -- published Thursday in the journal Science -- suggests the offspring of giant carnivorous dinosaurs, megatheropods like Tyrannosaurus rex, thwarted speciation.

Ecologically speaking, these fast-growing teenage theropods likely operated like distinct species, hogging resources that might have supported several smaller species.

"Dinosaur communities were like shopping malls on a Saturday afternoon -- jam-packed with teenagers," study leader Kat Schroeder said in a press release.

"They made up a significant portion of the individuals in a species, and would have had a very real impact on the resources available in communities," said Schroeder, a graduate student at the University of New Mexico.

Though T. rex gets most of the attention, the ecosystems of the Mesozoic era, which stretched from 252 million to 66 million years ago, featured a diversity of large-bodied dinosaurs.

To better understand how species differed among small, medium and large dinosaurs, researchers analyzed the fossil records compiled at dozens of dig sites all over the world.

In total, scientists analyzed more than 550 dinosaur species. Within disparate dinosaur communities, researchers sorted species by mass and diet.

The sorting process revealed an obvious pattern. In communities where megatheropods were present, researchers found evidence of very few dinosaur species weighing between 200 pounds and one ton.

The analysis showed the evolutionary impacts of megatheropods changed over time.

"Jurassic megatheropods don't change as much -- the teenagers are more like the adults, which leaves more room in the community for multiple families of megatheropods as well as some smaller carnivores," Schroeder said.

"The Cretaceous, on the other hand, is completely dominated by Tyrannosaurs and Abelisaurs, which change a lot as they grow," Schroeder said.